The invention relates to a compound for poducing lectrodes and to a process for forming electrodes. More particularly, the invention relates to silver pastes or powders for the formation of electrodes on substrates made of glass, especially glass of the soda-lime type, such as those used for plasma display panels.
In order to define the problem better, the present description relates to the production of plasma display panels. Of course, the invention is not limited to processes for producing plasma display panels but applies to any type of process using materials of the same kind under similar conditions.
As known from the prior art, plasma display panels (hereafter called PDPs) are display screens of the flat type. There are several types of PDP, which all operate on the principle of an electrical discharge in a gas accompanied by the emission of light. In general, PDPs consist of two insulating tiles made of glass, conventionally of the soda-lime type, each supporting at least one array of conducting electrodes and defining between them a space filled with gas. The tiles are joined together so that the electrode arrays are orthogonal. Each electrode intersection defines an elementary light cell to which a gas space corresponds.
The electrodes of PDPs have the feature of being small in cross section (of the order of a few hundred xcexcm2), in order not to impede the viewing, and of being very long (of the order of one meter). The electrodes must be made from a material that is a good conductor, allowing electrodes to be produced with a resistance of less than 100 ohms. In addition, the material used must be able to allow lower-cost mass production. At the present time, two techniques are known for producing these electrodes.
The first technique is thin-film metal deposition, which may be carried out by sputtering or by vacuum evaporation. The metal layer generally consists of a copper or aluminium layer placed between two chromium layers, the metal deposition taking place over the entire surface of the tile. A photosensitive resin is then deposited, the resin being exposed through a mask. Next, the resin is developed, thus creating a mask on the metal layer. The metal layer is then etched by acid etching. Finally, the excess resin mask is removed. One advantage of this technique is that it is carried out cold. However, this technique has a number of drawbacks. This is because the process requires many manufacturing steps and metal deposition is fairly expensive. In general, the layers deposited by this technique have thicknesses of about 2 to 3 xcexcm. A variant of this technique consists in depositing successive layers in order to reduce the overall cost, but this creates uniformity defects on the electrodes.
A second technique is the deposition of a photosensitive silver paste. For this, a silver paste is used which consists of 50 to 70% of silver particles (or particles of another highly conducting metal), having a mean diameter of the order of 1 xcexcm, the particles being mixed with a powder of a glassy material (for example, a borosilicate) and bonded together by a photosensitive resin. The silver paste is deposited on the tile and then exposed using a mask.
The exposed paste is developed in water, and then the assembly is fired between 450xc2x0 C. and 580xc2x0 C. so as to vitrify the glassy material and remove the excess resin. Using the paste makes it possible to have electrodes which are relatively thick (conventionally, of the order of 10 xcexcm in thickness) with a reduced number of manufacturing steps. Moreover, one variant consists in depositing the silver paste directly by screen printing. Direct screen printing consists in depositing the paste through a mask, thereby eliminating the exposure step and saving on base material, but it remains limited in resolution to dimensions of the order of 100 xcexcm.
The use of silver paste for the PDP tiles is preferable to the use of thin-film deposition, firstly for cost reasons and secondly for electroconductivity reasons. However, in this specific application a problem arises, as illustrated in FIGS. 1 to 5. A layer 1 of silver paste is deposited on the substrate 2, exposed and then developed so as only to leave the paste forming the electrodes 3. During firing of the electrodes 3, diffusion 4 of silver atoms and/or ions into the substrate 2 occurs. After the firing, the substrate 2 has a yellow-coloured diffused region 5 below each electrode. An insulating layer 6 is then deposited, by depositing a powder or a paste of an enamel, for example an enamel based on lead borosilicate or bismuth borosilicate, which covers the electrodes 3 and substrate 2. The insulating layer 6 is then fired between 550 and 590xc2x0 C. However, during firing of the layer 6, there is significant diffusion, represented by the arrows 7, of silver into the insulating layer 6 which is in a fluid state during the firing. At the end of firing, electrodes 3 of slightly reduced cross section and surrounded by a diffusion region 8 are obtained. The diffusion region 8 is not conducting. The main drawback with this diffusion region 8 is its yellow colour which is to the detriment of the transparency of the tile which supports the electrode array(s), something which is particularly problematic when the tile is the front tile through which light has to pass.
The main object of the invention is to improve the screen-printing process of the prior art by reducing the firing temperature and/or by simultaneously firing the electrodes and the insulating layer, while reducing the yellowing of the substrate and of the insulating layer. The invention provides a novel compound of materials which solves this problem. The invention proposes to partly or completely replace the powder of glassy material with a metal powder whose melting point is below the firing temperatures used in the manufacture of a plasma display panel. The use of a meltable metal powder allows the conductivity of the electrodes to be increased while increasing the cohesion of the silver particles. Furthermore, the use of a meltable metal as binder after melting makes it possible to use resins which are not compatible with borosilicates, thereby reducing the diffusion of silver into the insulating layer.
The subject of the invention is a compound of materials for forming electrodes on a glass substrate, the compound comprising a powder of a conducting metal or alloy and a powder of a meltable metal or alloy.
Preferably, the melting point of the meltable metal or alloy is less than 580xc2x0 C.
According to various embodiments, the compound may furthermore include an adhesion promoter, for bonding the electrodes to the substrate, a resin and/or a photosensitive substance.
Preferably, the compound is a paste in which 50 to 87% of its mass consists of conducting metal, 3 to 30% of its mass consists of meltable metal, 2 to 20% of its mass consists of adhesion promoter and 8 to 35% of its mass consists of resin.
The invention also relates to a process for manufacturing a plasma display panel, wherein the compound of the invention is deposited in a pattern on a glass substrate, an insulating layer of a glass in the form of a powder or a paste is deposited and the whole assembly is heated to a temperature of less than or equal to 580xc2x0 C. The insulating layer is deposited as soon as the compound has been deposited in a pattern, without firing the electrodes beforehand.
The subject of the invention is also a plasma panel whose tiles are obtained by the process of the invention.